Hybrid electric vehicles (HEVs) can be selectively powered in an electric-only/electric vehicle (EV) propulsion mode using one or more electric traction motors. The traction motors can also assist an internal combustion engine in delivering torque to a set of drive wheels. Each traction motor is selectively energized by a high-voltage energy storage system (ESS). In return, the ESS may be recharged during vehicle operation by capturing braking energy during a regenerative braking event. The ESS may also be recharged when the vehicle is not running, for instance by plugging the ESS into a wall outlet or charging station.
An HEV typically runs in EV mode up to a threshold speed before transitioning to engine power. A power transmission transfers engine and motor torque to a transmission output member in a combination that is determined by the controller as a function of requested input torque. The transmission may contain various fluid-actuated clutches which selectively connect and disconnect members of one or more gear sets to thereby select between various transmission states, e.g., fixed gear modes, electrically variable or continuously variable transmission modes, and one or more EV modes.